Fluctuation Magnetoconductivity Research Articles

Scaling beyond 3D [formula omitted] in the fluctuation conductivity of YBa [formula omitted]Cu [formula omitted]O [formula omitted

Systematic measurements of the in-plane fluctuation magnetoconductivity in a YBa 2 Cu 3 O 7−δ single crystal are presented. Fields up to 500 mT were applied either parallel or perpendicular to the Cu–O planes. The data reveal the occurrence of Gaussian-fluctuation regimes far from the transition. Closer to T c , the results clearly show the occurrence of a genuine critical regime, where the exponent is consistent with predictions for the 3D XY universality class with model- E dynamics. Still closer to T c , evidence is found for a fluctuation regime beyond 3D XY. This new scaling can be interpreted as revealing an ultimate first-order character of the superconducting transition in YBa 2 Cu 3 O 7−δ .

The mixed state properties of superconducting YNi 2B 2C

YNi 2B 2C intermetallic superconductor has the fluctuation magnetoconductance showing the 3D scaling behavior. In the mixed state, due to this fluctuation effect, the vortices form a solid phase at low temperature and make a liquid phase near T c similar to high- T c materials. Moreover, the vortex solid becomes a lattice at low fields and the vortex glass transition is found for high fields of H⩾2 T while the liquid phase is narrow.

Gaussian, three-dimensional-XY,and lowest-Landau-level scalings in the low-field fluctuation magnetoconductivity ofBi2Sr2CaCu2O8

Systematic measurements of low-field fluctuation magnetoconductivity in a single crystal of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ are reported. Gaussian, critical, and lowest-Landau-level scalings are observed. In the Gaussian regimes, large intervals corresponding to low-dimensional fluctuations are evidenced. Far above ${T}_{c},$ effects of disorder produces a fluctuation spectrum characterized by a fractal topology. Decreasing the temperature, at first a homogeneous two-dimensional behavior is observed. Then, near ${T}_{c}$ a crossover occurs to a narrow three-dimensional (3D) mean-field regime. Still closer to ${T}_{c},$ a scaling consistent with the predictions of the full dynamic 3D-$\mathrm{XY}$ universality class is clearly evidenced. This genuine critical regime is destroyed upon the application of magnetic fields above a few mT. For fields above a certain limit and in large temperature intervals, fluctuation magnetoconductivity scales as predicted by the lowest-Landau-level approximation of the Ginzburg-Landau theory.

Fluctuation magnetoconductivity in Fe doped YBa 2Cu 3O 7−δ. Sign change for B and I parallel to the planes

The magnetoconductivity in Fe doped YBa 2Cu 3O 7−δ single crystals has been measured in the temperature region above T c where superconducting fluctuations are known to be important. For small Fe concentrations and B¦c we obtain excellent agreement with accepted fluctuation theories. However, at higher Fe concentrations this description seems to break down. For B ⊥ c we observe an anomalous dependence on the relative orientation of current and field, both applied along the ab-planes. For B¦I the magnetoconductivity is negative, as expected. For B⊥ I we observe a cross-over to a positive magnetoconductivity above about T c + 10 K.

Low field fluctuation magnetoconductivity in Bi2Sr2CaCu2O8: critical, Gaussian and LLL scalings

We report measurements of the conductivity transition of a Bi2Sr2CaCu2O8 (Bi-2212) single crystal. Several magnetic fields up to 200 Oe were applied either parallel or perpendicular to the measuring current. We derive H-T phase diagrams showing the regions of dominance of 3D-XY, Gaussian and LLL fluctuation regimes above the critical temperature.

Anisotropic fluctuation magnetoconductivity in Bi2Sr2Ca2Cu3Ox thin films

Measurements of the fluctuation magnetoconductivity as a function of both the magnetic field up to 13 T and the temperature in thin films of Bi2Sr2Ca2Cu3Ox are presented. The variation of the magnetoconductivity with the magnetic field strength is quadratic at temperatures distantly above the critical temperatureT c and changes to an entirely negative curvature nearT c . The latter behavior is attributed to an inhomogeneous critical temperature in the films. The results are analyzed in terms of recent theoretical models for fluctuation-enhanced magnetoconductivity in layered superconductors. The magnetoconductivity in a magnetic field oriented normal to the copper-oxide layers is dominated by the orbital contribution of the Aslamazov-Larkin effect. The magnetoconductivity in the parallel orientation is distinctly smaller and provides evidence for the corresponding Zeeman contribution. The latter measurement also indicates that the Maki-Thompson process may be insignificant in Bi2Sr2Ca2Cu3Ox.

Novel approach for the analysis of the fluctuation magnetoconductivity in YBa 2Cu 3O 7 thin films

We report measurements of the paraconductivity and of the fluctuation magnetoconductivity as a function of both the magnetic field up to 13 T and the temperature in thin epitaxial films of YBa 2Cu 3O 7. An analysis within the model for dirty-limit superconductors yields consistent values for the in-plane, the out-of-plane coherence lengths, and the quasiparticle phase coherence time. Although an estimation of the mean free carrier path indicates that YBa 2Cu 3O 7 is a clean superconductor, the appropriate clean-limit theory produces an unrealisticly short phase coherence time, around half the value of the transport scattering time, and fails to fit the paraconductivity data. We show that if the magnetoconductivity due to the orbital motion of the normal-state quasiparticles is taken into account, the Maki-Thompson fluctuation contribution at high temperatures is significantly smaller than predicted by both clean- and dirty-limit theories. This result suggests non-s-wave pairing in YBa 2Cu 3O 7.

Magnetoresistance measurements in high temperature superconductors as a sensitive probe for inhomogeneities of the critical temperature

A phenomenological model which considers the effect of a non-uniform critical temperature on the thermodynamic fluctuation contributions to the paraconductivity and to the magnetoresistance is presented. Assuming a Gaussian distribution of critical temperatures, only a small change of the paraconductivity, but a significant enhancement of the fluctuation magnetoconductivity is predicted in the vicinity of T c . The present model is compared with recent measurements of the magnetoresistance in thin films of (Bi, Pb) 2Sr 2Ca 2Cu 3O x. It is shown that an analysis of the magnetoresistance provides a sensitive method for the detection of a non-uniform T c and also an estimation of the width of the T c -distribution function.

Influence of a distribution of critical temperatures on the paraconductivity and the fluctuation magnetoconductivity in high-temperature superconductors

A phenomenological model which considers the effect of a non-uniform critical temperature on the various thermodynamic fluctuation contributions to the paraconductivity and to the magnetoconductivity is presented. Assuming a Gaussian distribution of critical temperatures, only a small change of the paraconductivity, but a significant enhancement of the fluctuation magnetoconductivity was found in the vicinity of T c. The shape of the magnetic field dependence of the fluctuation magnetoresistance is substantially changed, in particular the quadratic region of the low-field limit is shifted to smaller magnetic fields. The present results are compared with recent experimental findings in thin films of (Bi, Pb) 2Sr 2Ca 2Cu 2O x and consequences for an analysis of the dimensional crossover in the layered superconductor model are discussed.

Fluctuation Conductance and Magnetoconductance in Superconducting Aluminum Films

The excess conductance σ ′ and the magnetoconductance Δσ due to superconducting fluctuations have been measured at temperatures near superconducting transition temperature T c of aluminum films. The data are analysed in terms of a current theory of the Maki-Thompson (MT) correction of which final expressions for σ ′ MT and Δσ MT depend on the electron mean free path l but not on the electron phase-relaxation time. Experimental results agree well with the sum of the AL (Aslamazov-Larkin) term and the present MT term. The value of l obtained by a best fit to the data on Δσ is reasonable. However, it increases with increasing temperature.

Fluctuation-enhanced conductivity and magnetoconductivity of high-quality YBa2Cu3O7?? crystals

We present measurements of the in-plane resistivityρ ab of YBa2Cu3O7−δ single crystals withT c ≃92 K andρ ab (100 K)≃50 μω cm. The temperature dependence of the fluctuation conductivity and of the magnetoconductivity aboveT c is analyzed in terms of direct and indirect fluctuation contributions for layered superconductors. The combination of fluctuation conductivity and magnetoconductivity allows to determine both coherence lengthsξ ab (0) andξ c (0) as well as the phase-relaxation timeτ ϕ of the pairs in an unequivocal manner. Evidence for clean limit type-II superconductivity in our crystals is given by large values of the mean free pathl ab ≫ξ ab (0).